The typical electronic regulator modifies the excitation applied to an alternator's field winding so as to hold the alternator's output voltage close to a desired mean value. This is usually achieved by periodically pulse-width-modulating the excitation voltage applied to the field winding so that the "on" time of the excitation voltage varies over a given regulation cycle (each regulation cycle may typically last for about 20 milliseconds). Where a larger alternator output voltage is called for, the "on" time of the excitation voltage is increased.
As described in more detail below, switching the excitation voltage off and on causes relatively large and undesired voltage steps to appear in the output of the alternator that is sensed by the regulator. These steps, plus other undesired transient voltages, are conveniently applied to a filter (typically, an R-C filter) prior to being processed by the regulator. If the filter is not able to quickly recover from the switching-induced voltage steps. improper regulation can occur. One undesirable result of such improper regulation is an apparent reduction of the regulation frequency from 50 Hz (typically) to 25 Hz. This change in frequency can manifest itself in visible headlight flicker, an obviously objectionable result.
As mentioned above, the cause of this problem is the inability of the regulator's filter to quickly recover from the switching-induced voltage steps. This recovery should occur during the regulator's minimum "on" time (a minimum interval when excitation is always being applied to the alternator's field winding) which may be as small as 1 millisecond. Thus, a filter with a fast time constant is called for. On the other hand, the filter's time constant needs to be relatively large to effect good regulation under all operating conditions. These conflicting requirements can give rise to compromises in the designs of the filter, and thus result in less than satisfactory regulation under all conditions.